High-resolution regional climate model experiments with a target area around Tokyo metropolitan area

Tokyo metropolitan area is located in the eastern part of Japan. The population in the area is over 34 million. The urbanization affects atmospheric environments in the lower troposphere. For example, surface air temperature is higher in the urban area than in the area surrounding the urban in night time, as such known as heat island. On the other hand, there are reports that precipitation is a little enhanced in the urban area in the afternoon basing on the observation and cloud-resolving atmospheric model simulations. Here, the signals of the enhancements are statistically significant but quite small. Thus, the significance of the signal should be further examined with various configurations of numerical simulations such as grid sizes. In addition, mechanisms of enhancements of precipitation should be investigated. In this study, ten-year climate simulations with a target season of August have been performed with 1- and 4-km grid sizes. In addition, three types of sensitive experiments have been performed (Total 6 experiments). In Type-I, a single layer urban canopy model named as the square prism urban canopy (SPUC) scheme developed by Aoyagi and Seino (2011) has been used. In Type-II, the urban area is artificially modified into grass area. In Type-III, only roughness in the urban area is considered.

The urban heat island phenomenon was clearly reproduced by a comparison between experiments of Type-I and -II. Increase of precipitation in the afternoon was not significantly reproduced by 1-km experiments, but cloud amount was a little enhanced around Tokyo. By thermal effect (difference between Type-I and Type-III), a turbulent mixing layer developed and low-pressure perturbation and the related weak convergence also developed on the inland side of Tokyo in the afternoon. By the thermal effect, the urban area environment become a little dryer. On the other hand, by dynamical effect (difference between Type-II and Type-III), a land-sea breeze was significantly weakened and affected the generation of clouds.

We are calculating regional climate model simulations under global warming environments. The results of future climates experiments would be added in the presentation.